A subset of the field of ergonomics, dealing specifically with computers and technology, and the more general field that encompasses usability. HCI focuses on the relationship between the user and the technology, often from a psychological perspective.
The study of the psychology and design principles associated with the way humans interact with computer systems.
The interaction between people and any computer-based system or product.
How users interact with computers and how to design hardware and software that is easy, quick and productive for people to use.
A discipline concerned with the design,evaluation and implementation of interactive computing systems for human use and with the study of major phenomena surrounding them. (HCI Bibliography, http://sigchi.org/cdg/cdg2.html#2_1)
Any way people and computers communicate with each other. Today most people communicate with computers via keyboards, pointing devices like the mouse and speech recognition software; most computers communicate with people via visual displays of text and graphics and audio output of sound, music and synthesized speech. The Human-Computer Interaction field of computing is focused on making computers easier to use by making this communication faster and more natural for people. For example, researchers are working on giving computers the ability to recognize gestures and facial expressions and engage in dialogue comparable to human conversation.